Summary:

Overview: The last major conflict of the Gallic
War, Alésia was the last stand of Vercingetorix. Vercingetorix
defeated the Romans at Gergovia September 52 B.C.E., Vercingetorix attacked the Romans with his cavalry near Divio (modern Dijon),
but afterwards had to flee towards
Alésia. Caesar laid siege on
Alésia and Vercingetorix with about a force of 70,000. In addition to the 50,000 men inside the fortress, Vercingetorix gathered a large relieving force to assist in a breakout (estimates of this relieving force vary from 100,000 to 250,000 men.) After a number of failed attempts to relieve the fortress from both outside and from within, Vercingetorix surrendered. He was taken to Rome and executed six years later.

The Gaulish city Alésia has been the place where the final battle between the Romans and the Gauls ended.
After a long hunt in which he 'advised' the helping towns to burn their crops, Vercingetorix sought refuge in this city in August 52
B.C.E., accompanied by about 80,000 - 95,000 warriors.
The about 60,000 Romans (10 or 11 legions) led by Caesar, builded a long, high, fortification around the city ("contravallation") so no one could escape.
Because of a possible relief of the town by a so-called "armeé de secours" the Romans later also build a second, outer, fortification ("circonvallation") to be protected against the Gaulish liberation fighters.
Those fighters indeed attempted to liberate Alésia. The Atrebati (led by Commios), Arverni (Vercassivellaunos) en (H)Edui (Eporedorix and Viridomar) brought together an incredible
300,000 men, which couldn't defeit the Romans.
After a siege of 2 months Vercingetorix handed himself to the chiefs of the tribes to let them do whatever was wise:
to kill him or hand him over to Caesar.
They decided Vercingetorix had to surrender to Caesar and so he did, September 52 B.C.E. Vercingetorix
was then taken to Rome as a slave where he spent the rest of his life in prison.
Six years later he was strangled in the arena during a feast in honour of
Caesar.

Vercingetorix, bronze statue in Alésia. Note that his moustache is absent on many other images, like
on the coin below right.

The "Archéodrome de Bourgondie"
has made a 100 m. wide reconstruction of a part of the contravallation, based on Caesar's "The Gallic War".
From left to right: wooden pointed stakes, 2 deep ditches (filled with water), wooden branches and hight towers.

The small stakes on the foreground were not just stakes but the precursors of the present mines.

They were sharpened and put in a small ditch which was covered with leafs; horses couldn't see and therefore not avoid them.
After that were upright stakes conceiled with branches, slanting hedges and 2 deep ditches.

An overview on the upright stakes conceiled with branches (top)
and the slanting hedges.

Arial view over the reconstructions.

Another view which shows the depth of the ditches.

A View from the side of the defenders, Vercingetorix on the foreground, on the last day of the siege.
After the disastrous attacks that were made by the "security army", the defenders lost all hope.
This attitude appeared to be typical Celtic; to fight with lots of
courage but to loose all hope when things start to go wrong.

After defeating our [the Roman /Teutonic] cavalry, Vercingetorix withdrew his
men that he had placed in front of the camp, and started to march towards
Alésia, which is the capital of the Mandubii, and ordered his soldiers to transport the luggages out of the camp and to follow him
swiftly. After the luggages were taken to a nearby hill and given in custody of two legions,
I [Caesar] chased the enemy as long as the duration of the day allowed to and killed about 3.000 people of the back of their army; the following day
I set the camp near
Alésia. After examining the location of the city and after terrifying the enemies, as
cavalry, which was the sector they trusted most, had fled away, I [Caesar] exorted the soldiers to work and started to build a trench around the city.

Caesar - Gallic War- Book VII, 69 - Alésia (Alise-Sainte Reine)

The actual stronghold of Alésia was set atop of a hill, in a very lofty situation, apparently impregnable save by blockade. The bases of the hill were washed on two separate sides by rivers.
In the front of the town a plain extended for a length of about three miles; on all the other sides hills were surrounding the town at a short distance, and equal to it in height. Under the wall, on the side which looked eastward, the forces of the Gauls had entirely occupied all this intervening space, and
had made a ditch in the front and a rough wall six feet high. The perimeter of the siege works which
we [the Romans] were beginning had a length of eleven miles. Camps had been pitched at convenient spots, and twenty-three forts had been constructed on the line.
In these picketts guards were posted by day to prevent any sudden attempt for an
escape; by night the same stations were held by sentries and strong garrisons.